1. Field of the Invention
The present invention relates to a phase-change memory device. More particularly, the present invention relates to a lower electrode in a chalcogenide memory cell. In particular, the present invention relates to a lower electrode with increased packing density.
2. Description of Related Art
As microelectronic technology progresses, the need has arisen for new data retention schemes. One such data retention scheme is the chalcogenide phase-change technology. Typically, a phase-change memory device includes a polysilicon lower electrode, also known as a xe2x80x9cmatchstickxe2x80x9d.
One challenge of forming a lower electrode in a phase-change memory cell is to shrink the cell size while not losing alignment tolerances. FIG. 16a illustrates the alignment tolerance 1 of a volume of memory material 2 over a lower electrode 3. There is required to be sufficient alignment tolerance 1 that only a first contact 6 of memory material 2 is made by one vertical section 7 of lower electrode 3. Further, the dimension of the volume of memory material 2 must be sufficient to assure a substantially complete overlay contact of vertical section 7, but have the alignment tolerance that is needed.
FIG. 16b illustrates the situation where a smaller lower electrode 4 is used and alignment tolerances are insufficient to prevent a second contact 8. If a double contact occurs or is likely to occur, several processes may be carried out in order to cause one side of lower electrode 4 to be what is called a xe2x80x9cdead matchstickxe2x80x9d. FIG. 16b illustrates the situation where the alignment tolerance has failed to make only a first contact 6. If both sides of the lower electrode 4 are conducting, than an inadequate memory operation will occur when there is misalignment as shown in FIG. 16b. To allow such misalignment, and still achieve sufficient memory operation, one previously disclosed method is to selectively dope and make conductive vertical section 7 and not vertical section 4, thus to create an undoped or dead matchstick 4.
After the formation of a recess in a substrate that exposes an active area, a conformal introduction of lower electrode material is required. Lower electrode material is typically polycrystalline silicon. The conformal introduction of lower electrode material that is polycrystalline silicon may follow conventional introduction techniques known to those skilled in the art including chemical vapor deposition (CVD) techniques. Thereafter, a dopant is introduced into the polycrystalline silicon to adjust the resistivity, in one aspect, to lower the resistivity of the material. A suitable dopant is a P-typed dopant such as boron introduced. From the combination of polysilicon and dopant, a silicidation process is required to form a silicide of the lower electrode. This process typically is a doping, a first anneal, a wet strip, and a second anneal.
After proper doping and fill into the trench, a planarization step is required to remove any horizontal component of the lower electrode. Thereafter, a modifier material must be introduced into a portion of the lower electrode material to combine and/or react with the lower electrode material near the top to form a different material. The modifier is introduced to raise the local resistance of the lower electrode material. By modifying a portion of the lower electrode material, the resistivity at that modified portion may be changed. Because the modifying material is of a higher resistivity, the lower electrode may not provide sufficiently suitable ohmic contact between the lower electrode and the volume of memory material for a desired application. In such cases, modifying material may be introduced into the lower electrode at a depth below the exposed surface of the lower electrode. For example, a lower electrode of polycrystalline silicon may have polycrystalline silicon at the exposed surface and a modifying material at a depth below the exposed surface. Additionally, barrier materials may be added to prevent cross-contamination between the chalcogenide material and the lower electrode.